There is known a foam molded product which is manufactured by foaming and fusing particles of a foamable synthetic resin in a foaming-in-place mold composed of two portions. These products are widely used as shock absorbing materials, containers, etc., since they are light in weight, are easy to handle, and have a good property of absorbing shock.
As the starting particles have very thin skins, however, the surface of the molded product has merely a film formed by the skins of the particles joined together like a tortoise shell. As it is not a uniformly thick film, the surface of the product cannot be said to be satisfactorily high in strength. The product is, therefore, unsuitable for some modes of use, for repeated use, or for washing with water. The rubbing or striking of something else against the surface of the product is very likely to cause the peeling of the film off its surface, or the depression of its surface. Moreover, the known product cannot necessarily be said to have a beautiful surface, since the foamed and fused particles form a pattern like a tortoise shell, or concavities in the surface of the product.
Attempts have been made to improve the drawbacks of the known product which make it useless under certain circumstances, as hereinabove stated. They include laminating a film of an unfoamed synthetic resin under heat on the surface of a molded product, or binding such a film to its surface with an adhesive, as well as melting the surface of a molded product under heat to form a film thereon. According to another attempt, a vacuum molded product is placed about a foam molded product to form a film on its surface.
According to still another attempt, an unfoamed sheet of a synthetic resin is first placed in a mold, and after the mold has been closed, particles of a foamable synthetic resin are charged into a cavity enclosed by the sheet, and an ordinary foam molding process is thereafter followed, so that the sheet may form a film on the surface of a molded product.
Further attempts are proposed in French Patent No. 1,571,274 and Japanese Patent Application Laid-Open No. 171924/1983. The French Patent discloses a foam molding process in which particles of a foamable polystyrene resin in a foaming-in-place mold are exposed to a relative pressure of 2 to 5 kg/cm.sup.2 (which corresponds to the pressure prevailing when they are heated to a temperature of 140.degree. C. to 150.degree. C.) for 10 seconds, for which vapor having a relative pressure of 5 to 10 kg/cm.sup.2 is employed, and immediately thereafter, the pressure is lowered to a level of 0 to 1.5 kg/cm.sup.2 (which corresponds to the pressure prevailing when they are heated to a temperature of 105.degree. C. to 110.degree. C.), and is maintained at that level. This process is claimed to be capable of making a molded product having a density which is 10 to 20 times higher in its surface portion than in its inner portion. The Japanese Patent Application disclosed a process in which the surface of a foam molded product made in a customary way from particles of a foamable polypropylene resin in a foaming-in-place mold is melted by heating at a temperature of about 200.degree. C. for a period of 30 seconds to form a film while the product stays in the mold.
The work of laminating or bonding a film of an unfoamed synthetic resin is, however, very difficult to accomplish without causing the wrinkling of the film. This is particularly the case with a molded product having a complicated shape. The melting of the surface of a molded product to form a film thereon has the drawback that the particles in its surface are difficult to melt uniformly and are likely to form a corrugated surface. In either event, and also when a vacuum molded product is employed to form a surface film, the manufacture of a final product requires a complicated process having two separate stages.
The process relying upon an unfoamed sheet of a synthetic resin placed in a mold has the drawback that firstly, its proper placement is no easy task, and that secondly, its wrinkling, for example, disables the formation of a neatly finished surface. This is particularly the case when the mold has a complicated cavity surface to make a molded product having a complicated shape.
The process disclosed in the French Patent has a number of problems, too. Although it does not contain any relevant description, it will be proper to understand that the particles forming the surface portion of a molded product do not melt, but retain their original shapes, since the particles as a whole are apparently heated to a temperature which is at least 50.degree. C. lower than the temperature of about 200.degree. C. considered as the melting point of the polystyrene resin. Accordingly, it is apparent that the molded product still has a surface having a pattern like a tortoise shell, or concavities formed by or between the individual particles, though its surface portion may be considerably higher in density that its inner portion. The surface portion has only a limited thickness, insofar as it is only the foaming pressure applied into the closed mold that is relied upon for achieving the higher density of that portion. The process precludes the use of any preliminarily foamed particles having a high degree of expansion, and cannot, therefore, be employed for making any product having a light weight which is an important characteristic of any ordinary foam molded product of a synthetic resin. The exposure of the particles to vapor having a high pressure, and immediately thereafter to vapor having a low pressure complicates the foam molding operation. Moreover, a high level of skill is required for carrying out the process successfully, since it is often likely that the hardening of the surface portion may occur immediately after exposure to vapor having a high pressure and prevent any satisfactory permeation of vapor to the inner portion.
The melting of the surface of the molded product as proposed by the Japanese Patent Application is likely to result in the failure to form a uniform film having no discontinuous portion on the surface of the product, or the formation of voids between the film and the underlying foamed and fused particles, due to the air staying in the particles after replacing a part or all of a foaming agent during the process of their foaming and fusing. Moreover, the proposed method can form only a film having a limited thickness.